can't print more decimal of pi [duplicate]

Answer 1

Like so:

#include <iomanip>
#include <iostream>

std::cout << std::setw(15) << pi_18 << std::endl;

The width modifier only affects the next formatting operation, so if you want to format multiple numbers, you have to repeat it before every one. Check out the full documentation of format specifiers.

Answer 2

You could use the precision method:

cout.precision(15);

This allows you to define the precision only once. You don't have to repeat it like with std::setw()

For more information see: http://en.cppreference.com/w/cpp/io/ios_base/precision